This invention relates to a pneumatic rotary grinding tool in which pressure air supply and exhaust passages in the tool housing communicate with air inlet and outlet ports on a motor unit, and which comprises a safety guard detachably mounted on the housing in a protective disposition relative to a grinding wheel attached to the motor spindle.
The invention intends to solve the problem of how to prevent use of a grinding tool of the above type when the safety guard is not properly fitted. Using the tool without having the safety guard fitted means an increased risk of injury for the operator and other people in the vicinity of the tool, because there is always a potential danger involved in the use of a rotary grinding tool. As is well known a grinding wheel exposed to excessive inertia forces at overspeed conditions might suddenly burst into a number of most dangerous high speed particles.
A correctly designed safety guard properly mounted on the tool offers a good protection against such a danger.
Another, more obvious potential danger of a rotary grinding tool is of course the tremendous cutting ability of a normally running grinding wheel which could cause severe casualties if, for example, the tool slips and the operator looses control of the tool. A safety guard partly surrounding the grinding wheel is effective to some extent in preventing accidents of this type too.
A previously suggested solution to this problem is described in U.S. Pat. No. 4,103,460. In the rotary grinding tool shown in this patent the safety guard forms an end closure for the tool housing. Thereby, the safety guard not only serves as a closure for a pressure air relief opening through which the pressure air supply passage in the tool housing is connectable to the atmosphere, but forms the only limitation to axial displacement of the motor unit. This means that should an attempt be made to activate the tool by opening the throttle valve when no safety guard is mounted there is a great risk that the entire motor unit will "disassociate itself" from the housing. Accordingly, this known power tool brings a new problem, namely, that if the tool is unintentionally activated the motor is expelled from the housing by the air pressure. This might be hazardous to the operator who runs the risk of being hit and hurt by the motor unit.
Linked to this direct safety problem is a secondary safety problem which may arise if the motor-speed governor unit is expelled from the housing and got damaged and/or exposed to dirt. Damage and/or dirt may jeopardize a correct speed governor operation and, thereby, cause a hazardous overspeed.
The above problems are solved by the power tool design according to our invention.
A preferred embodiment of the invention is below described in detail with reference to the accompanying drawing.